Optical spectroscopy is a suitable technique to study the functional and structural properties of large biomolecules in solution without damaging the samples. In particular, measurements of time-resolved fluorescence may give important information on the tertiary structure of proteins. Furthermore, the study of fluorescence depolarization as a function of time is a very common method to follow the dynamics of the local domains and subunits which form the quaternary structure of oligomeric enzyme. The analysis of fluorescence decays in terms of continuous distribution of lifetimes seems to be an appropriate approach to describe the dynamics of proteins which ranges over an enormous time scale (from 10 ps to 10 ns). The results obtained for several proteins are reported and discussed. The data provide further confirmation of the correlation which exist between heterogeneous fluorescence decays and a hierarchy of many conformational substates in proteins.